1. Field of the Invention
The present invention relates to a film (more particularly, a material constituting a film) crystallization apparatus and method, and a phase shift mask. The apparatus and method generate a crystallized film, e.g., a polycrystal or monocrystal film (semiconductor film), by irradiating a film to be subjected to crystallization processing, e.g., a polycrystal or amorphous film (e.g., a semiconductor film), with light rays having their phase modulated by using a phase shift mask, i.e., a phase shifter.
2. Description of the Related Art
In a liquid crystal display (LCD), there is known a switching element which controls a voltage applied to pixels of the LCD or a thin film transistor (TFT) as a semiconductor element which is used in a driver circuit driving the switching elements. As materials of a primary part of the transistors, there are roughly known amorphous silicon (a-Si), polycrystal silicon (poly-Si) and single crystal silicon (single-Si).
Since the electron mobility of the single-Si is higher than that of the poly-Si and the electron mobility of the poly-Si is higher than that of the a-Si, the single-Si and the poly-Si have a high switching speed when applied to the transistors and there are advantages, e.g., an increase in response of the display, a reduction in a design margin of any other component and the like.
When peripheral circuits such as a driver circuit or a DAC incorporated in the display are constituted by the transistors, these circuits can be operated at higher speed.
Although the poly-Si is composed of an aggregate of crystal grains, its mobility is lower than that of the monocrystal silicon, and it has a problem in irregularities of a grain boundary number which enters a channel when a transistor is reduced in size.
Therefore, there have been recently proposed various kinds of methods which generate polycrystal silicon or, more preferably single crystal silicon with a large particle size.
As one of such methods, there is known “phase control ELA (excimer laser annealing)” by which a film to be crystallized is irradiated with a pulse laser beam through a phase shift mask with the phase shift mask being set parallel to the semiconductor film. This technique is described in detail in “Surface Science Vol. 21, No. 5, pp. 278–287, 2000” incorporated herein as a reference. Further, a similar technique is also disclosed in Japanese patent application No. 2000-306859 (laid open in Nov. 2, 2000).
This phase control ELA is a method by which an inverse peak pattern that a light intensity becomes minimum, e.g., substantially zero at a phase shift portion, i.e., a shift boundary is generated by alternately shifting a phase of the light ray passing through a mask pattern of the phase shift mask by 0 and π, a minimum intensity position of the inverse peak pattern is set to an area (crystal nucleus) which is solidified first, and the crystal is grown from that position in the lateral direction (lateral growth), thereby providing the crystal nucleus with a large particle size at a specified position. Here, the inverse peak pattern means one inverse chevron type intensity pattern such as that a minimum intensity is positioned at a peak of the minimum intensity and the intensity is gradually increased with distance from this peak. An intensity pattern that this inverse peak pattern is continuously or discontinuously positioned is referred to as an inverse peak waveform pattern in this specification.
A shape of the phase shift mask, a distance between this mask and a surface of a film to be crystallized (which will be referred to as a semiconductor surface for the convenience sake), an incident angle distribution of a laser beam to the semiconductor film are set in such a manner that an ideal light intensity profile can be obtained on the surface of the film.
This phase control ELA must hold the phase shift mask so as to be close to the semiconductor surface as much as possible or, typically it must be held with a gap of approximately several μm to several hundred μm in order to obtain the light intensity profile corresponding to the phase shift mask. If the both members are close to each other in this manner, a part of the semiconductor film is ablated and shattered due to irradiation of the laser, thereby contaminating the phase shift mask.
Therefore, there is a problem that the light intensity pattern is degraded with a use and crystallization does not correctly advance.
In order to attain accurate irradiation, a position of the semiconductor surface must be adjusted in an optical axis direction with respect to the phase shift mask. Since a gap between the phase shift mask and a semiconductor substrate is very small, there is also a problem that a sensor system for the positional adjustment or a light flux used for detection cannot be inserted.